The present invention relates to the improvement on photomasks used for the fabrication of semiconductor integrated circuits, and more particularly to the improvement on photomasks suited to the case where patterns formed thereon are inspected while using electron beams as probe means.
The inspection of a pattern on a photomask with an electron beam includes successively irradiating the surface portions of the photomask under consideration with the electron beam and detecting a signal representative of pattern related information derived from the mask. As such a signal can be used backscattered electrons, secondary electrons, absorption current, etc. The checking of an object surface condition utilizing backscattered electrons, secondary electrons, absorption current, etc. derived from the object upon irradiation thereof with an electron beam is well known. For example, U.S. Pat. No. 3,381,132 discloses the analysis of a specimen utilizing secondary electrons and backscattered electrons, and U.S. Pat. No. 3,549,999 discloses the testing of integrated circuits utilizing secondary electrons. Since the intensity of the above-described information signal varies depending upon the kind of the mask surface material, it is possible to inspect the configuration and size of the pattern by detecting such a signal while successively scanning the mask surface with the electron beam.
The electron beam scanning process would require provision for preventing the mask surface from being charged by the scanning electrons. One approach for that purpose is proposed by U.S. application Ser. No. 925,791 filed July 18, 1978 by Koichiro Mizukami and Masatoshi Migitaka, now U.S. Pat. No. 4,256,778, and assigned to the assignee of the present application (corresponding to German patent application No. P2832151.9 filed July 21, 1978). This patent application proposes to dispose a transparent conductive layer on the mask surface or to make the mask substrate of a transparent conductive material. The electron beam is irradiated under a condition in which the conductive layer or substrate is connected to a predetermined potential such as a ground potential. Various examples of such a conductive mask are shown in FIGS. 1A to 1C. In the figures, reference numeral 1 designates a transparent substrate of insulating material such as glass, 2 a transparent film of conductive material, 3 a mask pattern or light shielding film of chromium (Cr), and 4 a transparent substrate of conductive material.
FIG. 2A schematically illustrates a signal obtained when the vicinity of the pattern of the mask shown in FIG. 1A is scanned with an electron beam. The mask pattern defines that portion of the mask serving as a light shield when the fingers on the mask is transferred onto a resist, and is hereinafter referred to simply as "pattern." An actual signal which can be observed would include noises as shown in FIG. 2B. The increased beam scanning rate for reducing a pattern inspection time would require a correspondingly increased signal detecting speed. In that case, the noises contained in the detected signal would be enhanced, as shown in FIG. 2C, thereby rendering the discrimination of a difference in signal intensity due to the presence/absence of the pattern difficult and remarkably deteriorating the reliability of results. However, if the signal intensity is obtained with a greatly distinguishable difference between pattern and non-pattern portions, as shown in FIG. 2D, it can be expected that the presence/absence of the pattern may be correctly recognized even if large noises are contained in the detected signal. In other words, if a mask itself is constructed so that it exhibits a high contrast (the ratio of a pattern related signal intensity to a non-pattern related signal intensity, and vice versa, namely, the ratio of the higher intensity to the lower intensity) through the scanning by an electron beam, the signal detecting speed can be enhanced without deteriorating the reliability of detected results.
FIG. 3 shows the present inventors' experimental results in which the contrast of backscattered-electron signal derived upon irradiation with electron beams is measured for mask samples which have the same structure as that of FIG. 1A and comprise a film of indium oxide In.sub.2 O.sub.3 (the actually used material containing a small amount of Sn and having the In/O ratio slightly deviated from 2/3) of a thickness of d.sub.1 .apprxeq.2,000 A as the transparent conductive layer 2 and Cr films of various thicknesses d.sub.2 as the pattern film 3. It can be seen from FIG. 3 that the sample having the thickness of d.sub.2 =1,000 A exhibits the maximum contrast. But, the maximum contrast as low as approximately 1.1 cannot allow an increase of checking speed. A main factor for the low contrast is considered to be a contribution of the underlying conductive material to the backscattered-electron signal intensity resulting from the penetration of an incident electron beam throughout the Cr film 3. Though the thickness d.sub.2 of the Cr film increased up to a value at which backscattered electrons from the underlying material are negligible would provide a signal intensity indicating a difference in surface material, it has been found that the thickness d.sub.2 above 5,000 A is necessary under an electron beam accelerating voltage of 20 KV for that purpose. Further, such an increased thickness d.sub.2 of pattern film makes the formation of a finely defined pattern difficult. Though the required pattern thickness can be decreased if the beam accelerating voltage is lowered, there would be drawbacks such as the deterioration of S/N ratio due to the decrease of beam current and the instability of beam position.